The following is an excerpt.

Aluminum makes our modern lives work. Sigurd Aune’s PhD work could help things work even better.

Male scientist in a mechanical lab
PhD candidate Sigurd Aune has taken on a not entirely straightforward task. He aims to model the behavior of heat-affected zones (HAZ) in welded aluminum joints. Here he does the first quasi-static tests on his welded specimens. (Photos: Sølvi W. Normannsen)

Ever heard of HAZ or heat-affected-zones? They occur during the welding of aluminium joints and can reduce the strength of the metal. Which in turn can lead to failure and fatal collapse of structures.
«The strength reduction that occurs during welding is one disadvantage of aluminium», says PhD candidate at SFI CASA, Sigurd Aune.
He aims to create a simple and effective computer model that predicts the exact behaviour of the zones when welded joints are subjected to extreme loads.

LOCAL HEAT REDUCES STRENGTH

Robot-welding metal

Ideally, products and parts should have the same maximum strength all over.
«What happens is that within a few seconds during welding, the area closest to the welding line is exposed to more than 500 degrees Celsius. Thus, the local heat causes a local strength reduction. This is a significant challenge», Aune explains.
Compared to the base material, the strength reduction within the zones may be up to 20-40 per cent. However, this depends on several factors such as the welding technique and type, the weld line´s length, the geometry and chemical composition. Also, the HAZ will vary in size.

A SIMPLE SOLUTION TO A VERY COMPLEX PROBLEM

Sigurd Aune´s model should be accurate and easy to use for the average industrial designer and engineer. Also, the interaction between many different materials and joining techniques must be incorporated.

And finally: the model must not be too computationally demanding. Most likely, he will focus on one welding technique and one material at a time.
«So, basically, we look for a simple solution to a very complex problem. Not exactly a straightforward task, but very rewarding», he says.

A MULTITUDE OF POSSIBLE END-USERS

Aluminium makes our modern lives work. It is the hottest of all goods in the metal industry, and you´ll find it in anything from mobile phones to beer cans, jetliners, and cars. The combination of mechanical properties such as low density, high strength, good ductility, and corrosion-resistance fuels the demand.
Aune´s work may be particularly relevant for the aluminium industry, car manufacturers, aircraft, and construction. However, as the CASA technology is generic, there could be a multitude of other possible end-users out there also battling the challenge of welded joints.

THE ROAD GOES VIA THE FINITE ELEMENT METHODMesh of welded aluminium specimen

The road to his model goes via the Finite Element Method (FEM). This is a favourite method among construction engineers, as it subdivides an extensive system into smaller, simpler parts – the finite elements. The automotive industry uses this method extensively to optimize its designs. The increasing use of FEM and numerical simulations are also gradually reducing the industry´s need for expensive experimental testing to document the crash safety of new vehicles.

A DIFFERENT APPROACH: LARGER AND ROUGHER

Sigurd Aune points at the welded line on one of his aluminium test specimens.
«We know that the HAZ can vary from 1 to 2 centimetres from this line. As the material´s properties will vary in these areas, we will put up a mesh with only one or two elements here».

Test specimen welded aluminium

His approach is not novel, it is just different. Previous work has been done with smaller elements. Aune makes it larger and rougher.
«I take it a step further. There is always this compromise between quick and effective and accurate», he says.

 

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ONE STANDARD DOES NOT FIT ALL

The PhD candidate assumes that numerical simulations and FEM enable more efficient and accurate design tools. Eurocode 9 (EC 9) is the European standard that describes how to design aluminium structures exposed to quasi-static loads. It keeps the producers on the safe side, but conservative designs do not always fit all purposes.

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«WE MUST KEEP IT SIMPLE»

Sigurd Aune´s work is limited to one of the most potent alloy combinations, the 6xxx-series. It is commonly used by the automotive industry.

The numbers of possible influencing parameters are kept to a minimum.«We must keep it simple. The more parameters, the harder to identify their influence respectively. Thus, one can quickly draw some wrong conclusions. The base for our approach is that the weakest point, the HAZ, sets the standard for the whole joint».

THE VIRTUAL LABORATORY FOR ALUMINIUM ALLOYS

Aune will use SFI CASA´s Virtual Laboratory (VL) for aluminium to develop the new virtual test procedure. He has started to build a mesoscopic model. The model simulates the strength and ductility of the HAZ exposed to different loading at the millimetre-scale. (Ductility is a mechanical property that enables a metal to be permanently deformed or pressed into another shape without immediately breaking). This model is quite precise, according to Aune.
The challenge is to take the next step: to develop a reliable macroscopic model that simulates the HAZ- behaviour at the metre-scale.
«I hope to get a draft of the model up and running during this spring», he says.

«So, we look for a simple solution to a complex problem»

ENGINEERING BEATS PROFESSIONAL GOLFING

A year and a half have passed since he embarked on the PhD at SFI CASA. He confirms the rumours that he actually could have been a professional golf player instead. In 2014, he became the Norwegian Junior champion. And 2nd place? Viktor Hovland, who is ranked 3rd in the most recent world golf rankings.
«I was offered a US scholarship combining college golf and studies. The downside was that it was economic studies only, and I wanted to study engineering. So, it wasn´t for me».

«NOW, I ACQUIRE CUTTING-EDGE EXPERTISE»

According to Aune, SIMLab and CASA offers a unique learning environment. He says that the group´s professors are outstanding and that he feels privileged to be surrounded by researchers interested in the same topics as he is.
«Besides, I can use 4 years to study things that I would never get a chance to do, if already working in the industry. Now, I acquire cutting-edge expertise. This is definitely the right place for me to be».
Sigurd Aune´s main supervisor are Professor Arild Holm Clausen.  Co-supervisors are Professor Magnus Langseth and Associate Professor David Morin.

Male researcher looks at computer screen
«Suppose we succeed in developing new procedures within the Virtual Laboratory and thus make physical tests superfluous. In that case, many people in the automotive and other industries will be happy».